This invention relates to methods and systems for cellular telecommunications, and more particularly to a multi-line telephone terminal system that enables multiple analog terminals to communicate via fixed cellular terminals supporting analog and digital voice channels.
Cellular radio telephone systems provide for communication among mobile stations and fixed telephone networks without requiring expensive wiring. Typically, a cellular system includes cellular transceivers or terminals that enable calls to be transmitted to and received from the network. Standard analog phone terminals can be interfaced with fixed cellular terminals so that a call can be set up at or received from an analog terminal via a fixed cellular terminal, avoiding the need for wiring to and from the analog terminal. The fixed cellular terminal alerts the analog terminal when an incoming call is received, causing the analog terminal to ring.
Several systems have been proposed for interfacing a standard telephone to a standard fixed cellular terminal. For example, U.S. Pat. No. 4,718,080 discloses a system in which, after a telephone number is dialed at a standard terminal, a SEND signal is simulated and the digits are transmitted to the cellular transceiver. Dialing sequences can also be entered at the standard terminal to control features of the standard terminal such as volume control. U.S. Pat. No. 4,775,997 discloses a system in which a SEND signal is transmitted to a cellular terminal after the last digit has been dialed at the standard terminal. The system determines whether the last digit has been dialed based on the elapsed time or count of digits dialed after a certain digit has been dialed. U.S. Pat. No. 4,959,851 discloses a system which enables predetermined dialing sequences consisting of leading and trailing function digits to be entered at a standard terminal to select features of a cellular terminal.
Conventional systems such as those described above use analog channels to transmit voice calls, FAX data and modem data to and from an analog terminal via a fixed cellular terminal. These types of systems employ standards such as the AMPS standard for analog modulation. In such systems, voice calls, FAX data and modem data are represented as analog signals.
It is desirable to provide a system supporting digital channels, enabling more voice calls in a given frequency bandwidth. A new standard, IS-54, has been produced by the Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) to support both analog and digital voice channels. According to this standard a "dual mode" cellular terminal is provided that supports both analog and digital voice channels. Voice coders are provided to code and decode voice data. Control signals and supervisory messages are communicated between the cellular terminal and a base station across a Fast Associated Control Channel (FACCH) if the base station is operating on a digital voice channel or through the use of blank and burst Frequency Modulated (FM) data if the base station is operating on an analog voice channel.
A dual mode system supporting digital and analog channels can transmit voice data over a digital channel using voice coders, but conventional modem data or FAX data cannot be transmitted over digital voice channels. This is because conventional modem data and FAX data consist of tones which the digital channel voice coders are not optimized to reproduce accurately. To transmit modem data and FAX data across a digital channel, conventional modems would have to be replaced with modems that produce data in a form that can be sent over the existing Time Division Multiple Access (TDMA) digital channel structure.
Since there is currently no provision for sending data over the TDMA digital channel in the IS-54 standard, the cellular terminal can automatically request or be notified to request an analog voice channel to handle the conventional modem or FAX data. If the cellular terminal continues to operate on a digital voice channel with modem or FAX data, the call will likely fail or will have sub-standard performance.
One way of overcoming this problem is for the analog user of the fixed cellular terminal to have a dedicated analog channel assigned to it from which it will receive or to which it will send modem or FAX data. This solution is cumbersome and costly, as it will necessarily set up calls as analog, even for digital voice capable calls, resulting in a waste of bandwidth.
Analog modem and FAX tones are similar in the way that they are sent at the beginning of a transaction. The tones are part of a recognition and negotiating exchange between the two end devices, according to the Committee Consultative International Telegraphique et Telephonique (CCITT) recommendations for modem and FAX protocols. The cellular terminal is able to detect the presence of these tones, even on a digital channel. This is advantageous in that the cellular terminal may initiate the request for an analog channel on its own without requiring the user to be involved or know what type of transaction is to take place.
Current analog FAX protocols have a long sequence of tones that can be detected by the cellular terminal. These are repeated in the case of a failure for as long as thirty seconds before the fax is sent or is determined to be a failure. This allows plenty of time for the cellular terminal to detect the tones and for the analog request transaction with the base station to take place. This is not the case for analog modem data protocols, however. They are very short in duration and have no overall retry attempts. This forces the user to become involved and initiate the analog request manually, prior to the call.
Thus, there is a need for a cellular system that permits a cellular terminal to automatically request an analog channel before a FAX call is initiated or to be notified to request an analog channel before a modem call is initiated, without actually having a full-time analog channel assigned.